With the tremendous interest in the development of solar energy, the design and development of solar energy collectors has proceeded in the last few years at an ever expanding rate. One overall limitation that has been encountered is in the cost of such devices. In the case of flat plate collectors, not only the cost and size of such devices has been a limitation but their efficiency is relatively low at best for a number of reasons. Suffice it to say it has been determined of late that the solar concentrator which in its simplest form is a parabolic or parabolic section mirror in combination with a collector medium located in a vessel at the focus or the line of focus of the mirror, has many advantages over the flat plate collector. One serious disadvantage, however, to the solar concentrator is that for any significant efficiency to occur, the parabolic mirror used must be of near optical quality with respect to its form and dimensional accuracy and stability. Of course, in the production of reflectors for light sources where only one reflector is required, the various methods such as electro-forming are available to produce a highly accurate mirror.
It has been recognized that if it were possible to form a conventional back surface glass mirror to the suitable parabolic shape at a minimum cost, the basic problems of the solar concentrator may be met and solved. Heretofore, composite mirrors have been produced including for example honeycomb support structures, or electro-formed or electro-deposited mirrors with front surfacing. None of these approaches have provided a satisfactory result. Another factor of great significance is that if a simple self supporting rear surface glass mirror can be achieved, the total weight as well as cost of solar concentrators can be reduced. Another factor mitigating in favor of the rear surface glass mirror is that a rear surface mirror is less subject to damage and may be cleaned to remove dust as simply as washing a window.
One limitation which has been encountered in attempts to form rear surface glass mirrors has been the lack of accuracy using conventional production techniques.
Under the auspices of the National Aeronautical & Space Commission, a significant improvement in the production of mirrors from domestic or home grade rear surface glass mirror blanks has been achieved employing the precision grinding of a foamed glass backing followed by pressure distortion and adhesive bonding of a thin glass mirror to the surface of the foam glass backing. This again has required optical grinding of the backing.
Non-planar glass optical elements have been produced in the past by glass sagging into a female cavity due to the weight of the glass or by positive forming pressure to the upper side of the glass blank.